This invention is in the field of reducing or eliminating acid rock drainage from sulfidic iron containing rocks and acidic mine waste tailings. Acid rock drainage (formation of sulfuric acid and related acids from natural air/water oxidation processes on various materials) is a common phenomenon from mining and leaching of various metallic and non-metallic minerals such as iron-containing sulfidic materials. These sulfidic materials include tailings, overburden, discarded waste rock and unmined exposed rock. Acid rock drainage causes severe pollution problems throughout the world.
There have been various attempts to render these sulfidic materials non-reactive which include partially converting pyrite and pyrrhotite into an oxide structure so that each iron sulfide particle is coated with an iron oxide film, microencapsulation of pyrite by artificial inducement of iron phosphate coatings, the coating of exposed surfaces with various polymeric materials, and the formation of manganese dioxide coatings on pyrite surfaces. These methods of treating materials have been at best partially effective and economically unattractive.
U.S. Pat. No. 5,587,001 (DeVries, Dec. 24, 1996) describes a method for reducing acid rock drainage from sulfidic iron-containing rock by contacting the rock with an aqueous solution of manganate ion at a pH between 6–13. This treatment reportedly creates a manganese oxide layer on the iron-containing sulfidic rock. The process in the U.S. Pat. No. 5,587,001 requires pH 6–13 at all times during the treatment, preferably a pH greater than 10. U.S. Pat. No. 5,587,001 also requires that permanganata color be maintained during the treatment. This condition often requires high dosage of manganate ions for treating reactive tailings because a considerable amount of manganate ions are dissolved in solution and react with other ions before reaching the sulfide surface. U.S. Pat. No. 5,587,001 also requires that the sulfides contain a significant concentration of iron bearing minerals so that the reaction between iron bearing sulfides and permanganate ions can be sustained. Several dissolved metals undergo precipitation reactions at pH>12. Precipitated metal hydoxycomplexes coat the sulfides, thus preventing the desired electrochemical reaction.
U.S. Pat. No. 6,086,847 (Thompson, Jul. 11, 2000) discloses a process for reportedly preventing acid rock drainage of metal-bearing rocks comprising contacting a sulfidic iron-containing rock with an acid passivating agent which comprises at least one alkaline earth metal to produce a combination; contacting the combination with manganate ions and a base and maintaining the pH of the system between 11 and 13.5.
The waste rock naturally yields very low acidic pH in the range of 1–4. To raise the pH and maintain it at a higher level than is naturally found (such as the pH required by the process disclosed in U.S. Pat. Nos. 5,587,001 and 6,086,847) requires high dosage of neutralization agents (for example, lime/caustic soda). This is not economically and technically viable. Also, at high pH (above about 11.0), gypsum (CaSO4) and MgSO4 precipitate on the sulfide and complete coating of desired materials cannot be achieved. Improved and cost effective treatments are necessary to treat iron containing sulfidic minerals to prevent or minimize the natural oxidation of these materials to form acids.